In a MAN, a user normally leases bandwidth based on transmission rate of Synchronous Digital Hierarchy (SDH), i.e., bandwidth level of the Virtual Channel-Trunk (VC-TRUNCK) on the MAN transmission device channel side. For example, at present the bandwidth leasable to users is categorized into the levels of VC-12, VC-3 and VC-4, among which the bandwidth value of VC-12, 2.048 Mbps, is the smallest one. If a user desires a bandwidth less than 2.048 Mbps, he has yet to lease a bandwidth of VC-12 alone since this is already the smallest leasable bandwidth for users, resulting in waste of system resources. Similarly, if the desired bandwidth level is between VC-12 and VC-4, the user may either lease a VC-4, resulting in waste of system resources as well because the desired bandwidth level is less than VC-4; or lease more than one VC-12, which will also lead to waste of system resources because, when the desired bandwidth can not be divided exactly by 2.048 Mbps, there must be a VC-12 providing more capacity than the requirement of the user.
To avoid waste of resources caused in the above situations and make more effective use of the bandwidth capacity of a MAN, a sub-rate transmission method for data services has so far been proposed. With this method, as shown in FIG. 1, Ethernet data packets from different input ports can be transmitted through one same VC-TRUNCK, then separated at the terminal transmission device and transmitted to respective output ports. The Ethernet data packets herein are also called data frames. In this way, data of different users can be transmitted through the same VC-TRUNCK, thus effectively avoiding waste of resources and improving utilization of bandwidth.
To differentiate the data which are transmitted through the same VC-TRUNCK but come from different input ports and to forward them to respective output ports, each data frame is identified in the prior sub-rate transmission method. Specifically speaking, by taking use of IEEE (International Electric and Electronic Engineering) standard 802.1 P/Q, the whole MAN is divided into a plurality of Virtual Local Area Networks (VLANs) among which mutual access is disablement, and each user is assigned with one or more VLANs. In this way, data frames in the same VC-TRUNCK can be differentiated according to the VLAN Identifier (VLAN ID) which is unique in the entire MAN. And the terminal transmission device, by referring to the pre-stored corresponding relationship between the VLAN IDs and output ports, is able to determine the output port for each of the data frames transmitted through the same VC-TRUNCK so that the data frames can be transferred correctly to their respective output ports. Obviously, it is possible to incorporate the priority defined in the IEEE Standard 802.1 P and assign different priorities for VLANs so as to realize Quality of Service (QoS) to a certain extent.
FIG. 2 shows an Ethernet data frame format with VLAN ID conforming to IEEE 802.1 Q. As shown in FIG. 2, a data frame is composed of a destination address field, a source address field, a 802.1 Q header label field, a length/type field, a data field and a check field. Among them, the 802.1 Q header label field includes a Tag Protocol Identifier (TPID) and Tag Control Information (TCI), the combination of which is called a VLAN Tag, with VLAN ID being a component of TCI. At present, the VLAN ID defined in IEEE 802.1 Q has only 12 bits, so that at most 212 VLANs, i.e., 4096 VLANs, can be accommodated in a whole MAN. With the expansion of the MAN, it is obvious that 4096 VLANs can not satisfy the service demand, thus restricting the development of the MAN. In addition, since the assignment of VLAN ID for each user must be made by unified planning within the whole MAN, the operation is rather complicated, bringing inconvenience to users.